How can I define Lambda Expression for an interface with more than one method?

Question

I can not define a lambda expression for an interface with multiple methods shown in the code below.

interface Interface{
    public void st1();
    public String ra(String p);
}

I can define a lambda expression for a single-method interface.

interface Interface{
    public String ra(String p);
}

public static void main(String[] args) {
    Interface r = (x)->{
        return x;
    };
    String getValue = r.ra("string");
    System.out.println(getValue);
}

But how can I define a lambda Expression for multiple methods?

Answer 1

You're thinking about functional interfaces. Anonymously defining a class with a lambda expression is a syntactic sugar that's only available for interfaces with a single method. If you have more than one method, you'll have to resort to a good old fashioned anonymous class:

Interface r = new Interface() {
    public void st1() {
        // do something;
    }

    public String ra(String p) {
        return p;
    }
}